Remove Lex::text_string_is_7bit

Our current grammar in sql_yacc.yy uses LEX_STRING to return TEXT_STRING and NCHAR_STRING terminal symbols from the tokenizer, and additionally uses Lex->text_string_is_7bit to know a difference between 7bit and 8bit strings (for optimization purposes).

This approach is error prone. Changes in the grammar that require more look-ahead can put Lex->text_string_is_7bit out of sync from bison variables ($1, $2, $3 etc), so for example Lext->text_string_is_7bit already corresponds to $2 instead of expected $1.

A safe approach would be to return the LEX_STRING and the corresponding 7/8 bit flag as a single structure like this:

struct Lex_string_with_metadata_st: public LEX_STRING

{

  bool m_is_8bit;

public:

  void set_8bit(bool is_8bit) { m_is_8bit= is_8bit; }

  // Get string repertoire by the 8-bit flag and the character set

  uint repertoire(CHARSET_INFO *cs) const

  {

    return !m_is_8bit && my_charset_is_ascii_based(cs) ?

           MY_REPERTOIRE_ASCII : MY_REPERTOIRE_UNICODE30;

  }

  // Get string repertoire by the 8-bit flag, for ASCII-based character sets

  uint repertoire() const

  {

    return !m_is_8bit ? MY_REPERTOIRE_ASCII : MY_REPERTOIRE_UNICODE30;

  }

};

and use this structure for TEXT_STRING and NCHAR_STRING.

The problem was revealed by valgrind in the bb-10.2-compatibility branch when extending this rule:

sp_proc_stmt_return:

    RETURN_SYM expr

  ;

to

sp_proc_stmt_return:

    RETURN_SYM expr

  | RETURN_SYM /* from a procedure */

  ;

Before making changes in the grammar we should fix this problem.